Display system for seismic data

ABSTRACT

A DISPLAY SYSTEM FOR PRODUCING A VISUAL RECORD OF ELECTRICALLY RECORDED SEISMIC DATA USING A PHOTOCONDUCTIVE MEDIUM. THE SYSTEM USES A CATHODE-RAY TUBE TO CONVERT THE ELECTRICAL SIGNALS TO VISIBLE LIGHT FOR WRITING THE SEISMIC DATA ON THE PHOTOCONDUCTIVE MEDIUM. THE SYSTEM INCLUDES MEANS FOR SPATIALLY LIMITING THE ELECTROSTATIC CHARGING OF THE PHOTOCONDUCTIVE MEDIUM PRIOR TO WRITING THE SIGNALS ON THE PHOTOCON-   DUCTIVE MEDIUMS. THE SYSTEM ALSO INCLUDES A MEANS FOR DEVELOPING THE CHARGE PATTERNS IN A LIQUID BATH SUCH THAT RAPID ACCESS IS ATTAINED, ALLOWING ONLINE INSPECTION OF THE PROCESSED DATA.

O i United States Patent [111 3,588,911

[72] Inventors Robert R. Luke [56] R f e Cit d n R b S L d UNITED STATESPATENTS ars a o mson, ugar an I Kenneh Thomson. Pasadena. 3 ofTex'2,976,107 3/196] Kle n et al 346/33X [2H Appl No 700,197 3,025,1233/1962 Klein 346/33X [22] Filed Jan. 24,1968 3,158,433 1 H1964 Alexanderetal 346/33X [45] Patented June 28, 197! Primary ExaminerBernard Konick[73] Assignee Shell Oil Company Assistant ExaminerGary M. Hoffman NewYork, N.Y. Attorneys-Theodore E. Bieber and J. H. McCarthy ABSTRACT: Adisplay system for producing a visual record [54] P ES Z SEISMIC DATA ofelectrically recorded seismic data using a photoconductive rawmg medium.The system uses a cathode-ray tube to convert the [52] U.S. Cl....346/74, electrical signals to visible light for writing the seismic dataon 340/155, 346/33, 346/1 10, 355/2 the photoconductive medium. Thesystem includes means for [51] Int. Cl G0ld 15/06, spatially limitingthe electrostatic charging of the photocon- GOld 9/l2, GOlv l/24 ductivemedium prior to writing the signals on the photocon- [50] Field ofSearch 346/74 (P), ductive medium. The system also includes a means for(CRT), (ES), (ESX), (ESB), 33 (Seis.), l 10; 355/2 (inquired); 179/1002(CRT); l78/6.6 (lnquired); 340/155 (Display) developing the chargepatterns in a liquid bath such that rapid access is attained, allowingonline inspection of the processed data.

PATENTED M28191:

SHEU 10F 2 FIG.

INVENTORS:

R. R. LUKE M. M. ROBINSON K. THOMSON ZM THEIR ATTORNEY PATENTEDJUN28|9H3588.911

SHEET 2 0F 2 SIGNAL SOURCE FIG. 3

R 24 CLOCK E V PULSE SWEEP D-A DEFLECTION GENERATOR REGISTER CONV.DRIVER I00) IOI) I02) I03) lO5'\ TRACE HIGH REGISTER VOLTAGE n=64 SUPPLY||0 D'A ON COL J P L I NG CONV' AMPLIFIER OFF INVENTORS:

R.R. LUKE M.M. ROBINSON TfiEE A gzigg gi ia 1 K. THOMSON BY: EL l c;%,f/i:;

4 THElR ATTORNEY DISPLAY SYSTEM FOR SEISMIC DATA Background of theInvention One of the basic tools used in the exploration of the earthfor possible petroleum deposits is seismic exploration. In seismicexploration an acoustic impulse is generated either by a chemicalexplosive, dropping of weights or other means. The acoustic impulsetravels through the earth and is reflected from various interfaces inthe formation and the reflections are received by geophones that areplaced at the surface in a predetermined pattern. The geophonestransform the received reflections into related electrical signals thatare then recorded either in digital form or in analog form. The recordedseismic signals or data are transmitted to a processing station wherethe data are converted to a visual form that can be used by geologistsin determining the possibility of petroleum deposits being present.

At a processing center the seismic data are filtered to improve thesignal-to-noise ratio and the individual signals are corrected for moveout and other abnormalities. The data may be processed either in ananalog form or in a digital computer or in a combination of analog anddigital systems. In either case, the processed seismic data exist onlyin a digital computer memory or in the form of an analog signal. Incontrast, the geologist requires a visual presentation in order tointerpret the data and decide on the possibilitiy of petroleum depositsbeing present. Thus, the problem is presented of transforming the datainto a visual presentation while preserv ing all the accuracy present inthe original data.

Over the years the visual presentation of seismic data has becomestandardized. For example, the time scale varies between 2 inches persecond to 12 inches per second. Since the total amount of recorded timein reflection seismic work runs to 7% seconds, this requires a recordbetween 15 and 84 inches long. Normally a record 42 inches long has beenused to display 7% seconds of recorded time. Further, the method ofrecording is normally a variable area recording although some seismicdata are visually recorded in the form of the single-trace orwiggle-type of signal. Finally the record should have printed thereonthe recording data blocks indicating the location wherethe data wasobtained and other information needed in interpreting the records.

A further requirement for visual presentation of data is a rapidprintout of the data. This is especially important in the case ofdigital computer processing of data since the computer supplies onecomplete seismic section approximately every minute on the average. Theseismic record usually corresponds to 24 geophones spaced in apredetermined pattern and the seismic section is made by playing outthese 24 trace record blocks side-by-side. Thus, it is seen that if itis possible to print out or transform the processed data to a visualpresentation faster than real time, a single display system can be used.

A further consideration in the transforming of seismic data to visualpresentation is the cost of the transformation. In view of the largeamount of data that are presently being processed, even a small savingsin the unit coast results in a large overall saving. In the past it hasbeen customary to play out the data on photographic film and thendevelop the film and make the required prints. This, however, has twodistinct disadvantages. Photographic film is expensive and does not lenditself to rapid access or online development. The latter can be veryimportant in digital computer processing. in that in the event of amalfunction, either human or machine, the situation may be detected muchsooner if the processed data are rendered visible sooner.

Summary ofthe Invention The present invention solves the above problemsby providing a display or play-out system for transforming processedseismic data to a visual presentation using photoconductive paper.photoconductive paper has come into wide use in recent years in documentcopiers and other duplicating equipment and has become a low costrecording medium. More particularly, the present invention uses a paperwhich may be electrostatically charged and then selectively dischargedupon exposure to visible light. The paper is then passed through adeveloper wherein toner particles adhere to the exposed areas to createa visual image of the processed seismic data. The system uses acathode-ray tube and associated circuits to convert processed data to avisual light form that can be used to discharge selected areas of theelectrostatically charged paper. The circuits associated with thecathode-ray tube are designed to sample the processed data at fixedintervals and display the resulting magnitude of the signals on the faceof the cathode-ray tube. The cathode-ray tube is designed to sample the24 traces of a seismic record at frequent intervals and print out acomplete seismic record during a single traverse of the tube across thepaper.

The system mounts the photoconductive paper in a fiat plane with thecathode-ray tube being mounted on a movable carriage. The carriage isdriven across the plane of the paper at a constant speed to generate thetime or depth axis of the plotted data. The rate at which the data arefed to the cathode determines the actual scale. Thus, the actual lengthof the record can be controlled by the seismic processing system andvaried to suit different conditions. The system also includes a coronacharger mounted on the carriage for electrostatically charging paperimmediately ahead of the cathode-ray tube. The corona charger isprovided with a variable mask to limit the electrostatic charging of thepaper to the exact width that will be exposed by the cathode-ray tube.This width also cor responds to the amount by which the paper is indexedafter each record is printed. Thus, all of the section receives the sameelectrostatic charge and a uniform section is generated.

Brief Description of the Drawings The above advantages and operation ofthe display system of this invention will be more easily understood fromthe following detailed descriptions of preferred embodiments when takenin conjunction with the attached drawings in which:

FIG. 1 simplified perspective drawing of the monitoring system;

FIG. 2 is a partial plan view of the cathode-ray tube and corona chargerof this invention;

FIG. 3 shows an end view ofa modified carriage including a simplifieddrive means for the carriage; and

FIG. 4 is a block diagram of the circuit for controlling the cathode-raytube.

Description of Preferred Embodiments As explained above, a majorrequirement for the display system is that it rapidly plays out theprocessed seismic data. In order to achieve the required speed of playout it is obvious that substantially all manual operations should beeliminated and the equipment automatically controlled. If this combination can be achieved while at the same time achieving reliability ofoperation, it is possible to provide a rapid play out.

Referring now to FIG. 1 there is shown a simplified perspective view ofthe display system of this invention. The display system uses aphotoconductive paper 10 for forming the visual presentation. Thephotoconductive paper 10 is preferably a paper which can beelectrostatically charged and then discharged in selected areas that aresubsequently coated with a pigmented toner to develop a visualpresentation or image of the seismic data. To achieve the required s eedand automatic operation the paper 10 is preferably mounted on largerolls, thus eliminating the time required to change the paper after eachseismic section is printed. The paper is supplied on a reel 11 and takenup on a reel 12 with the reels 11 and 12 being mounted on a suitableframework, not shown in FIG. 1. In addition, the take up reel 12 shouldbe provided with a suitable drive means that is controlled by theseismic data processing system in order that it may be intermittentlyoperated to present a new portion of the paper after each recording. The

portion of the paper on which the seismic data are recorded ismaintained in a relatively flat uniform plane, at least between thelimits shown by the upper line 13 and the lower line 14.

The seismic data are transferred to the paper by means of a cathode-raytube 24 that is mounted on a movable carriage IS. The carriage 15 isdriven in a direction normal to the advance of the paper 10. While thecarriage is traveling across the paper to play out the processed seismicdata, the paper 10 is maintained stationary. During the return of thecarriage 15 to its starting position the paper 10 is advanced to presenta fresh portion of the paper for recording the next record of seismicdata. The carriage 15 may be driven by means of an endless geared beltand a belt-seizing mechanism, not shown in HO. 1. The belt-seizingmechanism selects one side of the belt to cause the carriage to travelin one direction and the opposite side of the belt to cause the carriageto travel in the reverse direction. Thus, the belt 20 can be driven in asingle direction at a speed that is related to the time axis of theseismic data. When the term time axis is used, it refers both to thetime base of the original seismic recording as well as the correctedtime or depth base of the processed record. Since the velocity of soundnormally increases with the depth of burial, real time cannot be relateddirectly to depth of burial, but corrections must be made. Theconversion of the time base to a depth base is performed during theprocessing of the seismic records.

The endless belt 20 is driven by a motor 22 through a speed reduction 23and drive sheave 21 while the other end of the belt is supported by anidler pulley 27. While the speed reduction is shown as a simple beltdrive, obviously more sophisticated variable-speed drives can beincorporated. The drive motor 22 is preferably a servomotor that can becontrolled by the seismic data processing system or the speed reductionmay be controllable. This will then permit adjustment of the horizontalor time axis of the recorded data to any desired degree to obtain thedesired length of record. Normally, for reflection work on land theoriginal recording is approximately 71; seconds long in real time. At a5 to 1 scale the paper 10 should be approximately 42 inches wide toprovide a 40-inch record. The speed control of the motor 22 by theseismic processing system can be used to provide time depth correctionsduring the actual playout of the process seismic data. This wouldrequire the seismic processing system be pro grammed so that it willsupply a control signal suitable for controlling the servomotor 22 orspeed reduction.

Referring to FIG. 2, there is shown a simplified top view of the displaysystem. The face of the cathode-ray tube 24 is preferably ofarectangular shape and its sweep is controlled as explained below so thatthe 24 traces of a seismic section will be written in a single traverseof the carriage l5. Positioned adjacent to the cathode-ray tube 24 isthe corona charger 25 that consists of a suitable housing 26 having athin metallic strip 30 with a serrated edge to produce a number of sharppoints mounted therein. The strip 30 is coupled to a high-voltage powersupply, for example, 10 kilovolts, that effectively ionizes theatmosphere around the sharp points 30. The ionized atmosphere willcreate an avalanche of oxygen ions which will then electrostaticallycharge the paper 10 immediately ahead of the cathode-ray tube 24. Thecorona charger is provided with a mask 31 having a variable opening 32.The length of the opening 32 is adjusted so that it correspondssubstantially to the width of the 24 traces being recorded by thecathode-ray tube. The use of the mask 31 is an important feature of theinvention, since it limits the area of the paper 10 that iselectrostatically charged to the exact area required for the 24 traces.Thus, there is no overlap between the adjacent recordings on the recordand a uniform section is recorded. The length of the opening 32 can beadjusted by various means, for example, by cam means or stepper motormeans. The length of the opening 32 can also be manually adjusted.Likewise, a fine wire can be used in place of the thin metal strip 30 inthe corona charger.

Referring now to FIG. 3, there is shown an end view of a completedisplay system constructed according to this invention. Moreparticularly, there is shown a carriage 40 on which the cathode-ray tube24 is mounted in addition to the corona charging means. The cathode-raytube is connected by a long lead to a signal system 41 shown in FIG. 4and fully described below. The carriage is provided with a base member42 that rides on two tubular guides 43 and 44. Suitable antifrictionbearings are provided for supporting the carriage 40 on the tubularguides, as for example, ball bearing supports 47.

The carriage is driven along the tubular guides by means of a drivecapstan 46 that engages one sidewall of a tubular channel-shaped member45 that extends downwardly from the base carriage. The channel-shapedmember may be formed of V a center section and thin sidewalls that areattached to the center section. The use of thin sidewalls is preferred,since they will absorb some misalignment of the drive means. The drivecapstan is driven by means of a motor 50 and a belt and pulleyarrangement 51. Idler wheels 52 and 53 are positioned on opposite sidesof the channel-shaped member 45 to backup the thin sidewalls and insurea driving contact between the drive capstan and the sidewalls.Positioning means (not shown) are used to move the drive capstan from adriving en gagement with one sidewall to a driving engagement with theopposite sidewall. This will permit driving the carriage in eitherdirection. In addition, a compression spring may be placed at the endofthe guides to stop the carriage and reverse its direction. Thecarriage has considerable weight and requires considerable energy tostop and reverse its direction. Thus, the compression spring will absorbenergy as it stops the carriage and supply energy as it reverses thedirection of the carriage.

The tubular guides are supported from a base member 56 that in turn ismounted on two channel-shaped members 54 and 55. The channel-shapedmembers are secured to the main frame of the display system, not shownin FIG. 3. The unexposed paper is stored in a lighttight magazine 60which is secured in a lighttight manner to the main frame of the displaysystem. The paper supply roll 61 is mounted on a suitable chassis and aweb of paper fed through the idler roll 62. The idler roll 62 isprovided with a friction brake 63 for preventing excess paper from beingpulled from the paper supply roll. The web then passes over idler rolls64 and 65 having releasable clamping members 66 and 67 that are capableof firmly clamping the web to the idler rolls. Positioned between theidler rolls 64 and 65 is a metering roller 70 that travels between theposition shown and a dotted position 71. The combination of the idlerrolls and the metering roller provides a means by which a fixed amountof paper may be intermittently advanced by controlling the releasing ofthe clamp members and the movement of the metering roll between the twopositions.

The record advancing elements include adjustable means for changing thelength of paper that is advanced to correspond with the adjustments inthe length of the opening in the mask covering the corona charger. Theclamp 66 and its associated roller, the clamp 67 and its associatedroller 65 and the movable roller 70, constitute a linear differential.In operation, clamp 67 is energized, clamp 66 is released, and movableroller 70 moves upward from its reference position against a fixed stoptoward an adjustable stop, shown as 71 in the figure. As it moves itpulls paper from the roll 61. When the movable roller has reached theadjustable stop at position 71, clamp 66 is reenergized, and the movableroller 70 returns downward to its fixed stop, leaving a loop of paperbetween clamps 66 and 67. At the completion of writing and on the returnstroke of the writing carriage, clamp 67 is released, which allows thetakeup motor to pull out the loop of paper. The adjustable stop forroller 70 is a variable radius cam. When changes are made in the lengthof the opening 32 in the mask 31 over corona charger 25, correspondingchanges are made in the position of the adjustable stop for roller 70.The later changes can be made manually or by automatic means havingactuating elements responsive to changes in the corona charger window.

The paper web then passes over two idler rolls 72 and 73 that arepositioned above and below an opening 59 formed in the mask member ofthedisplay system. The paper passes over the backing member 74 which servesto support the paper in a relatively flat plane. As explained above, thecarriage includes a means for electrostatically charging the paper onlyin the area in which it will be exposed.

After the seismic data is written on the paper, it passes over idlerrolls 73 and 75 into a tank 79 of developer. The tank 74 is suppliedwith developer through a line 76 while the excess developer whichoverflows the tank or drains from the paper is drained from the space 78by means of a drain hose 77. The developer consists of a suspension ofcharged pigmented toner particles in a liquid. The toner particles arecharged with the same polarity as the paper so that they are repelled bythe unexposed areas of the paper. After the paper passes through thedeveloper, it passes over idler roll 80 that is provided with a squeegeetype wiper 81. The wiper removes the excess liquid from the paper webwith the liquid returning to the volume 78 where it can be drained andrecirculated to the developer tank. The wiper 81 may beelectrostatically charged to assist in removing toner particles from theportions of the record that are intended to be unmarked. If the wiper ischarged with the same polarity as the toner particles, the tonerparticles will be repelled by the wiper and removed from the paper.

After the excess liquid is removed from the paper web, it passes over aroller 83 disposed in a tank of fluid 82. The paper web after passingover the roller 83 passes over an idler roller 90 to takeup roller 91.The takeup roller 91 is driven by motor means 92 with the drive meansbeing capable of slipping when the paper is held stationary and takingup the slack when the paper is released. The liquid in the tank 82 issupplied by a line 84 with the excess being drained through a line 85 toa supply tank 87. The fluid in the supply tank 87 is circulated by meansof pump 86 to the tank 82.

An alternate arrangement to the above-described developing system wouldbe the use of a spray head to apply the developer to the exposed paper.The developer can be sprayed as a single solution or the two-part systemdescribed above may be used. With either system the spray head should bemounted adjacent the paper web and preferably where it passes over aroller. For example, the spray head could be mounted in tank 79 adjacentthe paper web where it passes over roller 75. The excess liquid willthen drain into the tank 79 and be returned to the storage tank.

The developer in the above system can be any commercially availabledeveloper using a liquid having a relatively high boiling point, as forexample, kerosene. The use of a liquid having a high boiling pointprevents an undue evaporation of the liquid and thus results in a moreuniform mixture of the toner particles and the liquid. This producesmore uniform records and prevents an undue concentration of the tonerparticles in the developer solution. In order to secure the fast airdrying characteristic necessary in a continuous developing process, thedeveloped web is passed through the liquid in the tank 82 which liquidis a relatively low-boiling point liquid, as for example, Freon 113.Thus, Freon 113 mixes with any liquid developer that remains on thepaper web and causes the liquid to be evaporated with the Freon 113.This thus provides the rapid drying characteristic necessary in anycontinuous process, while maintaining a uniform developer solution.Since the Freon 113 is separate from the liquid developer, evaporationof the Freon 113 will not result in an increased concentration ofdeveloping particles in the developing tank. Thus, the overall qualityof the record is greatly improved.

As explained above, the cathode-ray tube 24 is swept in only onedirection with the Z-axis or beam being brightened at appropriate times.Thus, if the sweep of the beam is repeated at a relatively rapid ratewhen compared with the forward advance of the carriage the resultingimage will appear as a continuous seismic trace. in order to obtain avariable area record the Z-axis or beam brightening will, of course, beon for a finite time and then turned off. This will provide avariablc-area record, while if the beam was merely turned off and on, itwould provide a truce or what is referred to as a wigglc record.

A conventional cathode-ray tube having a phosphor screen face plate mayhe used. in place ofa conventional cathode-ray tube a tube wherein thephosphor screen is coupled to paper It) by means of an optical system,as for example, fiber optics, is preferred. This type of tube eliminatesthe tendency of the light to spread as it passes through theconventional glass envelope. In one satisfactory system an 8inchcathode-ray tube was used and swept at a rate of 4,000 Hz. At this sweeprate the data of each trace will, in a 24-trace seismic record, besampled every 250 microseconds. The normal frequency range of seismicsignals is 20 to I00 Hz. This will insure a continuous trace that willmaintain the fidelity and accuracy of the seismic processing system.

Referring now to FIG. 4, there is shown a block diagram of the circuitfor controlling the sweep of the cathode-ray tube 24 and in additionactuating the Z-axis or beam brightening. To minimize the drift problemsthe sweep signal of the cathode-ray tube is generated by a digitalsystem and more particularly a clock pulse generator 100. This is afree-running oscillator whose frequency is accurately controlled as bymeans of a crystal. The output of the clock generator is cou- [tied toboth a sweep register 10] and a trace register 105, both circuits beingbasically counting circuits. The sweep register is designed so that itrequires 64 clock pulses to carry the cathode-ray tube beam across thespace allotted to each trace. Thus, the sweep register will be requiredto count to a total of i536 pulses. The sweep register is coupled to adigitalto-analog converter 102 that converts the output to an analogsignal. The analog signal from the circuit 102 is supplied to thedeflection driving circuit l03which in turn controls the voltagesupplied to the deflection yoke 104 of the cathode-ray tube. Thus, thedeflection circuit will cause the beam to sweep across the face of thecathode-ray tube at a rate controlled by the clock pulse generator. Thiscircuit will inherently rovide greater accuracy thanis possible with theconventional sweep circuit wherein the ramp voltage is generated in aconventional oscillator circuit.

Simultaneously with the advance of the sweep circuits the trace registeralso accumulates the pulses from the clock generator. The grace registersupplies a start or on signal to the on connection of a flip-flop 111.This signal is supplied at the start of each individual count of 64pulses. The output of the trace register is also supplied to adigital-to-analog converter 106 which converts the instantaneous countto a related analog signal. The analog signal is supplied to a comparingcircuit 113 which also receives ap analog signal representing theamplitude of the seismic trace at any particular instant oftime. Thecomparing circuit 113 compares the amplitude of the seismic trace withthe amplitude of the signal from the trace register. When the amplitudeof the two signals agree the comparing circuit opens the gat 114,turning off the flip-flop.

The signal from the flipflop is passed to a coupling amplifier 116 thatin turn controls the Z-axis or beam brightening sweep of the cathode-raytube. The beam brightening voltage is supplied from a high voltagesupply 120.

When the above circuit is operated, it is seen that at the start of arecording cycle the clock pulse generator will control the bearn of thecathode ray tube at a sweep rate of 4,000 cycles per second. Inaddition, the clock pulse generator will divide the sweep up into 1536increments so that each trace will consist of 64 individual increments.The pulses from the clock generator also control the beam brighteningcircuit. This is achieved by turning on the beam brightening by means ofthe flip-flop at the start of the portion of the beam sweep allotted toeach individual trace. Thus, the beam brightening circuit will be onuntil the amplitude of the signal from the trace register I05 equals theamplitude of the seismic trace. This will thus supply a variable-arearecord of the seismic trace.

From the above description it can be seen that the individual seismictraces will be recorded with an accuracy of I part in 64, or an accuracyof l.5 percent. Of course, each trace to be recorded will require itsown individual comparing and gate circuit to enable it to print thetrace at the proper time of the sweep of the oscilloscope. Thus, eachgate can only be opened for a maximum of one twenty-fourth of thecomplete sweep of the cathode-ray tube.

A wiggle or single trace line record can be produced by intensil'yingthe beam momentarily as each comparator circuit delivers its comparisonpulse. This can be accomplished by reversing the leads to the flip-flopso that it is normally turned off by the trace register and only turnedon for an instant when the comparator circuit delivers its comparisonpulse. When wiggle records are recorded, four overlapping sweepregisters can be used with each trace register to enable it forone-sixth of the time of the sweep register. Thus, one can recordoverlapping wiggle traces four times as large as the trace height whenvariable-area records are made. This is, of course, the normal seismicrecording procedure in the case of wiggle traces, since an overlappingof signals that are in phase tends to amplify or accentuate the signals,while out-of-phase or noise signals will be attenuated and decrease inintensity.

We claim:

I. A system for converting machine-processed seismic data to graphicform, said system comprising:

a frame member;

a photoconductive sheet in the form of a continuous roll;

means for supporting said photoconductive sheet in a flat plane on saidframe member;

carriage means movably mounted on said frame member in a plane parallelto said photoconductive sheet;

a cathode-ray tube having deflection and beam brightness circuitry;

means for mounting said cathode-ray tube on said carriage means adjacentsaid photoconductive sheet with the face of said cathode-ray tubesubstantially parallel to the plane of said photoconductive sheet;

a corona charging means, said corona charging means being mounted onsaid carriage means;

a drive means, said drive means being coupled to said carriage means todrive it in a first direction at a constant speed to generate the timeof depth axis of the seismic data and in a second direction to returnsaid carriage to its initial position; means for advancing said rollintermittently in response to said drive means moving said carriage insaid second direction; and, control means connected to said cathode-raytube, said control means including:

1. a source of clock pulses;

2. a first digital counting means connected to said source ofclqckpulses;

3. first digital to analog converter having the input thereof connectedto the output of said first digital counting means and the outputthereof connected to the deflection circuitry of said cathode-ray tubewhereby the beam of the cathode-ray tube is caused to sweep across thetube at a rate controlled by said source ofclock pulsets,

4. a second digital counting means connected to said source of clockpulses, a second digital to analog converter means connected to saidsecond digital counting means,

6. means for comparing the output of said second digital to analogconverter with the amplitude of the seismic data signal and supplying anon/off control signal to the beam brightness circuitry of saidcathode-ray tube when the output of said digital to analog converter issubstantially equal to said seismic data signal.

2. The system of claim I wherein said carriage travels in a directionnormal to the direction of advance of the photoconductive sheet.

3. The system of claim I wherein a mask is mounted between said coronacharger and said photoconductive sheet, said mask limiting the portionof the sheet being electrostatically charged to the area exposed by saidcathode-ray tube.

4. The system ofclaim l and in addition a developing means includingmeans for contacting the photoconductive sheet with a high-boiling-pointsuspension of toner particles and then with a relativelylow-boiling-point liquid that is miscible with the suspended liquid.

5. The system of claim 1 and in addition an energy absorbing meansdisposed to absorb the energy of the carriage traveling in one directionand return the absorbed energy to the carriage when the carriagereverses its direction oftravel.

LII

